An aerosol is a suspension of liquid or solid particles in a gas. Aerosols are used for a variety of purposes, including delivery of a substance such as a medicinal drug or an insecticide. In some cases, an aerosol is formed with a can or bottle that contains a propellant and a substance to be delivered. Some aerosols are considered to be harmful to the environment. Other aerosols are considered to be an efficient and cost-effective means to deliver a substance to a target area.